The present invention relates to a modular beam amplifier for use in generating high power beams of particles in systems such as a spacecraft propulsion engine, and more particularly to a lightweight and highly efficient spacecraft propulsion engine.
A typical ion linear accelerator using direct current (DC) voltage generators such as Cockcroft-Walton, Van de Graaff, or Pelletron circuits is described in U.S. Pat. No. 2,215,155. An insulated pipe containing a vacuum has a series of metal tubes or annular discs spaced at intervals down the pipe, each tube or disc at a progressively higher voltage. Ions are injected at the low voltage end and accelerated in stages by the electric field in the gap between each tube or disc. The acceleration is proportional to the total voltage between the low voltage end and the high voltage end of the pipe. The intermediate voltages for each tube or disc may be set by individual voltage generators, or there may be a single large voltage generator and the intermediate voltages set by resistors, connected as a series of voltage divider circuits. This linear accelerator will amplify or increase the energy of the ions from a lower energy when entering the accelerator to a higher energy when exiting the accelerator.
Previous approaches to generating very high DC voltages for linear accelerators have used Cockcroft-Walton circuits, described in U.S. Pat. No. 2,875,394, or Van de Graaff generators, described in U.S. Pat. No. 1,991,236, or the Pelletron generator described in U.S. Pat. No. 3,469,118. These types of circuits can generate DC voltages of thousands to millions of volts.
The Cockcroft-Walton circuit does not have any moving parts, but there are disadvantages in using it for generating high voltages, especially for spacecraft applications. For one thing, the DC voltage generated will have a ripple voltage, and the ripple voltage will get higher and higher as the number of diode/capacitor ladder stages increases. A high ripple voltage makes it difficult to focus a charged particle beam in a linear accelerator into a narrow particle beam. Most Cockcroft-Walton circuits will have 3 to 20 circuit stages. In the case where there are 20 circuit stages, the diodes and capacitors must have voltage ratings of at least 50,000 volts for each million volts of output. It is difficult and expensive to build or buy diodes and capacitors rated for that high of a voltage. In addition, the voltage regulation drops significantly as more ladder stages are added. This makes the Cockcroft-Walton approach heavy and expensive, which makes it inappropriate for use in spacecraft propulsion applications.
Van de Graaf and Pelletron generators are electromechanical mechanisms that require motors and moving belts or chains to generate high voltage, and they are impractical in generating more than a few kilowatts of power. These types of generators are not well suited for high voltage spacecraft power applications because of their weight and lack of long term reliability due to the continuously moving parts.
Other approaches to linear particle accelerators use AC voltages, typically at radio frequencies, as described in U.S. Pat. No. 3,133,227. The AC approach can generate extremely high voltage energy particle beams without the need to insulate against high DC voltages, but the existing implementations of this type of accelerator are too inefficient, heavy, and expensive to use in space propulsion applications.
There are also several types of existing electrostatic ion engine designs such as the gridded electrostatic ion thruster described in U.S. Pat. No. 4,825,646, the Hall effect thruster described in U.S. Pat. No. 5,845,880, and the field emission thruster described in U.S. Pat. No. 6,053,455. None of them operate with voltages above 105,000 volts, which limits their fuel consumption (specific impulse) performance. All existing designs use voltage converters or regulators between the main portion of the ion engine and the power source, which is usually a solar panel array, which increases the system weight and cost, and limits their system efficiency. In addition, all of the voltage converters or regulators are located in the main body of the satellite, resulting in heat buildup there which must be removed by thermal conductors or thermal radiators, especially for high power ion engines.
There is also a type of ion drive where the solar panels are directly connected to the ion drive grids as described by European Patent EP 0909894 A1. However, since grids are still used for accelerating the ions, their design suffers from increased grid erosion as the accelerating voltage is increased. Grid erosion is the major limit on the operational lifetime of gridded ion engines. Because of grid erosion, the number of grids is limited to only three or four.
Another type of ion engine is the VASIMR described in U.S. Pat. No. 6,293,090, which is an electromagnetic thruster which uses radio waves to generate a high temperature plasma and magnetic fields to form a nozzle. The latest embodiment requires the use of superconducting magnets, which increases the weight and requires large amounts of power for refrigeration to keep the nozzle magnets at a superconducting temperature.
It would be desirable to find a spacecraft ion engine that operates efficiently at high voltages without being overly encumbered by weight, cost, or cooling requirements.